1. Field of the Invention
The present invention relates to a head position correcting system for a disc storage unit such as a fixed disc unit and more particularly to a system for correcting a position of a head in the disc storage unit by closed-loop control in response to an off-track amount detected from servo information read out in a so-called data-surface-servo system in which recording information and so-called servo information are stored on a surface of a disc.
2. Description of the Prior Art
In a recently developed disc storage unit, not only reduction in diameter of the disc but also an increase in data storage capacity has been demanded, and therefore efforts have been made in the prior art to increase storage density on the surface of a disc. Recently, one major surface of a small disc about 3.5 inches in diameter has hundreds of information recording tracks, so that only the use of only a conventional open-loop control system has become not sufficient in order to control a transducer head or read/write head for writing and reading out data into and from a track, so that the head is positioned precisely to a desired track. Therefore, it has become necessary to employ a closed loop control system. That is, reference information or so-called servo information is previously written in the surface of a disc and this servo information is read out by the read/write head to detect a deviation of a position of the head from its proper position or a so-called off-track amount, so that the position of the head is corrected by the closed-loop control system in such a way that the detected off-track amount is within a predetermined allowance range. In the head position correcting systems of the type described above, use is made of a surface servo system in which all of one major surface of a disc is exclusively used to store the servo information. But, in this system, a precious area for recording general information on the surface of the disc is exclusively used for the storage of the servo information. As a result, the demand for increasing the storage density cannot be met. Therefore, recently, a data-surface-servo system in which the servo information is stored only in a limited area of the surface of the disc upon which general information is stored has been mainly used.
Since a precise control of a position of the head is based upon the servo information, in either of the control systems described above, a position at which the servo information is stored must be determined accurately on the surface of the disc Therefore, in the case of writing the servo information, a writing unit exclusively designed and constructed to write the servo information has been used. In this writing unit, the position of the head is accurately measured by, for instance, a laser beam or the like and after the head has been correctly located at a predetermined position, the servo information is recorded Of course, the recording of the servo information is accomplished after a plurality of discs are assembled in the disc storage unit. In this case, a head position can be accurately measured for a specific disc surface, for instance, a surface of the outermost disc of the stack of a plurality of discs, but an accurate measurement of a head position is not easy for a surface of the remaining discs between the outermost discs. The recording of the servo information by the exclusive write unit has a high accuracy, but the detection and control of a position of the head in relation to each track requires some time, so that it takes a substantial time for writing the servo information into hundreds of tracks on the surface of the disc.
Therefore, in a conventional mass-produced disc storage unit such as a fixed disc unit, the servo information is written only in one disc surface of a plurality of disc surfaces in the disc storage unit. Of course, mechanical positions of the respective heads in relation to a plurality of disc surfaces are not uniform in a strict sense. However, the setting of recording information tracks on the surfaces of all the discs and the tracking of the heads for reading or writing data are accomplished based on this single servo information, since it has been considered that it is not needed to write the servo information in all the disc surfaces.
In principle, there is no reason for limiting the servo information to the above-described single servo information, but practical experience of running the disc storage unit so far has shown the fact that there occurs a failure in writing or reading required data which is considered to be caused by an error in a head position. The result of the analysis of such a failure shows that the reading or writing failure hardly occurs in case of the surface of the disc on which the servo information is recorded, and that it frequently happens that a reading or writing failure occurs in the case of a surface of a disc on which the servo information is not recorded.
It is, therefore, considered that the reading or writing failure is mainly caused by mechanical problems, and more particularly that an inclination of a shaft of a spinning motor for rotating a disc and an inclination of a carriage for carrying and guiding the head in a direction of the movement of the carriage are delicately varied due to variations in ambient temperature or operation temperature or mechanical errors caused by the use of the disc unit for a long period of time. In the case of a fixed disc unit, such errors become several micrometers at the maximum when a track pitch is of the order of 30 .mu.m. Thus, it is understood that the errors become almost equal to a so-called gap portion which separates adjacent tracks from each other.